[time-nuts] Another "atomic" clock question

[Jim Lux]

>>> As Jim mentions in another post, you can run on the fundamental and the third (or 5th or 7th) and get a thermometer out of the delta between the two modes. The gotcha is that a change in load impedance will shift the frequencies unequally. That will give you an apparent temperature change.
>>
>> I already know about the fundamental and third trick, my question was if it could be done to AT-cut as well. I interpret your statement as yes, it does. I don't trust it to be perfect, but reasonable. Ideas for means to handle shift would be welcome.
>
> It was originally proposed by a very nice guy from Ft. Monmouth for use with AT cut resonators. I believe the paper is in the FCS proceedings from the mid 1980’s. The DOD kept rights to the technique and licensed it to a couple of oscillator companies.


Hmm.   SC cut, perhaps? (see the third reference down..

R. L. Filler and J. R. Vig, “Resonators for the microcomputer 
compensated crystal oscillator,” 43rd Ann. Symp. Freq. Contr., pp. 8- 
15, 1989.


there's also

The microcomputer compensated crystal oscillator (MCXO)

  Bloch, M. ; Frequency Electron. Inc., Mitchel Field, NY, USA ; Meirs, 
M. ; Ho, J.
The MCXO uses a novel technique to achieve temperature compensation 
without the use of ovens or conventional temperature-compensating 
components. The crystal oscillator in the MCXO, which is free to vary 
with temperature, operates on two modes simultaneously-the fundamental 
and the third overtone. Several advantages accrue because this method of 
temperature compensation does not resort to frequency pulling. The 
authors presents the details of how the MCXO operates and the details of 
the performance of the delivered systems
Published in:
Frequency Control, 1989., Proceedings of the 43rd Annual Symposium on
Date of Conference:
31 May-2 Jun 1989

Page(s):
     16 - 19
Meeting Date :
     31 May 1989-02 Jun 1989
INSPEC Accession Number:
     3685419

Conference Location :
     Denver, CO
Digital Object Identifier :
     10.1109/FREQ.1989.68853




But then,

Yoonkee Kim (from Ft Monmouth)
has a paper (DTIC ADA484423)
Aging of Dual Mode Resonator for Microcomputer Compensated Crystal 
Oscillator
Abstract— A Microcomputer Compensated Crystal Oscillator (MCXO) utilizes 
the dual c-mode excitation (fundamental mode and 3rd overtone (OT)) of 
an SC-cut resonator for self- temperature sensing and compensation. The 
long-term stability of the MCXO depends primarily on the aging of the 
dual mode resonator. When two modes age differently in time, the aging 
MCXO’s output frequency curve would shift with a tilt over its operating 
temperature range